Introduction

The preservation of digital information has become a critical issue for many organizations, not least the libraries and archives of the US Government. According to the National Digital Information Infrastructure and Preservation Program (NDIIPP), which is administered by the LC, information is being produced in greater quantities and with greater frequency than at any time in history. Furthermore, according to the NDDIIP, electronic media, especially the Internet, make it possible for almost anyone to become a "publisher". The program asks how will society preserve this information and make it available to future generations . Optical media has become the medium of choice for digital storage in many government agencies as well as many organizations within the private sector. It is therefore imperative that the archive capabilities of optical media be known.

In June 2004, the Information Access Division of the National Institute of Standards and Technology (NIST) and the Preservation Directorate at the Library of Congress (LC) agreed to perform a detailed investigation of the longevity of recordable Compact Disc (CD) and Digital Versatile Disc (DVD) media. This report outlines the procedural details of that investigation for estimating the life expectancy of information stored in CD-R, DVD-R and DVD+R, as well as DVD-RW and DVD+RW discs. The test procedure uses accelerated aging techniques and statistical analysis to estimate the life expectancy (LE) of current recordable DVD and CD media. This report also includes details of a ‘shelf-life' test of recordable CD and DVD media, where blank media are aged artificially and their recording properties examined as the media is aged.

Only the effects of temperature and relative humidity on the media are considered. The standardized life expectancy estimated using this model is defined for discs maintained at 25 ºC and 50 % RH, but can be applied to give an estimate of the life expectancy at any moderate storage conditions. Discs exposed to more severe conditions of temperature and humidity are expected to experience a shorter life. The test plan does not attempt to model degradation due to exposure to light, corrosive gases, contaminants, or mishandling, nor does it account for variations in the playback subsystem. Furthermore, the compatibility of the recorder and the media will have a critical effect on the LE of media, including very stable media since error rates immediately after recording will impact the time before failure. For this study, great care was taken to ensure compatibility between the recorder and the media, thus ensuring low initial error rates.

The ISO standard, ISO 18927:2002 ( Imaging materials – Recordable compact disc system -- Method for estimating the life expectancy based on the effects of temperature and relative humidity, first edition) , was used as a basis for the aging of the media and the analysis of data in this study, and thus some of the content of this paper will be familiar to those familiar with the ISO 18927:2002 standard. However, significant adaptations were required to ensure that suitable error rates and data points were obtained for recordable media. Initial testing, carried out in 2004 and 2005 indicated that the stress conditions and durations described in ISO 18927:2002 (minimum of 500 hours per aging cycle at increased temperature and humidity conditions) may be too harsh, with the result that very high error rates were obtained after just one aging period. An approach whereby the incubation length for each aging cycle could be altered, depending on the error trends, was adopted.

The joint NIST/LC optical media longevity test plan started in 2004 and it is expected that this investigation will be completed in 2006, after which results and analysis will be disseminated in a suitable publication, within the guidelines and legal restrictions agreed by NIST and the LC.

Technical Background

Recordable optical disc media contains an organic dye layer whose transparency can be altered either to absorb a laser beam or to allow the beam to pass through to a reflective layer behind the dye [1,2,3]. The nature of this organic dye is such that when the internal energies of its molecules reach a particular threshold, an irreversible chemical reaction occurs, and the dye layer loses its transparency. This property allows a high-energy beam to ‘write' data by burning ‘pits', in the form of dark marks, to the disc during recording. A low powered laser reads the data by either passing through the transparent dye layer (without causing any molecular change) to the reflective layer or by being absorbed by the nontransparent marks in the dye.

Due to the organic nature of the dye, degradation and breakdown of the transparent portion of dye layer will occur over a long period of time as a natural process. This process, which has its roots in chemical kinetics, can take several years in normal environment conditions [4]. Higher temperatures and humidity will accelerate this process by increasing the thermal and kinetic energies of the dye molecules.

The effect of these processes can be modeled using various techniques including the Eyring model, which has a theoretical basis in chemistry and quantum mechanics and can be used to model acceleration when many stresses are involved. For this study, the Eyring equation will be used to model the effect of two stresses, temperature and relative humidity, on the rate of a reaction or degradation, which can be related to the time-to-failure of the optical disc.

The Eyring model has found broad application and shall be the model for estimating life expectancies of Recordable DVD systems. The equation

t₅₀ = AT ^ae^ΔH/Te^(B+C/T)RH

where

t₅₀ is the time to 50% media failure;
A is the pre-exponential time constant;
T ^a is the pre-exponential temperature factor;
ΔH is the activation energy per molecule;
k is Boltzmann's constant (1,38 × 10 ^-23 J/K);
T is the temperature, K;
B, C are the RH exponential constants;
RH is the relative humidity, %;

was derived from the laws of thermodynamics and, in this form, can readily be seen to handle easily the two critical stresses of temperature and RH. For the temperature range used in this test, it is common practice to set a and C to zero [5].

The Eyring model equation then reduces to

t₅₀ = Ae^ΔH/kTe^(B)RH

Disc Burning Stations

For this investigation, each disc burning station (DBS) consists of a Pentium-4 processor workstation with the following specifications:

Intel Pentium-4 Processor 3.6 GHz w/ 800mhz FSB,
1GB DDR2-533 MHz dual channel RAM,
Seagate 120 GB 7200 rpm SATA hard drive w/ 8 MB cache,
Microsoft Windows XP Pro w/ doc and media,
Running Roxio Easy Media Creator 7, version 7.5.0.47 ENU.

The recording drives used in this study include the Sony DRU-510A, 700A , 800A, Pioneer DVR-A09 , Plextor PX-716SA and NEC ND -2510A .

More information about the DBS on the Procedure page.

Analyzers

For testing DVDs:
CATS SA300 DVD-R/RW PRO and CATS DVD+R/RW PRO from Audio Development are used to analyze the DVD media in this study.

For testing CDs:
CD CATS SA3 Advanced is used to analyze the CD media in this study.

More information about the Analyzers on the Procedure page.

Environmental Chamber

Home | Procedure | Definitions
Overview | Current Phase | Links
Collaboration | Results | Contact
---------------------------------------------------------
National Institute of Standards & Technology ©2005